While transitioning to a new website one of the biggest mistakes often made is creating a new site with differing site keywords. It's important to keep your keyword strategy consistent from one website to the next. While this might sound scary, all it means is that you should keep your content consistent from old to new site, at least during the transition period
If you're not familiar with the term Keyword, it is actually quite simple. This is something that defines your business and is used by others to find you. For example, if you own an Italian Restaurant in San Francisco that delivers food, you would expect people to search for 'Pasta San Francisco delivery' or 'Italian Delivery' to find you. These would be your keywords. They relate to your core business and are embedded into your existing site.
Search Engines want Consistency
You will want to keep these keywords consistent from old to new websites. Since search engines have likely already indexed your website, they will expect to see the same or similar content on the webpage when they index your site again. If the content is vastly different, this will hurt your rankings as your content is not relevant to what the search engine thinks should be there.
Assuming you own an Italian Restaurant, you would not want search engines indexing your site and finding information about a taxi service on your site. This would be irrelevant. You would want them finding similar content to what you had before. Small variations & optimizations are okay and event expected by search engines. When you change things drastically is when you can get yourself into trouble.